A prior art flash erasable and electrically programmable read-only memory ("flash EPROM") includes memory cells that individually can be programmed with a single bit of data. The data stored in a selected memory cell of the prior art flash EPROM can be read by a sense amplifier. The data stored in the memory cells of the prior art flash EPROM can be electrically erased once programmed.
The prior art flash EPROM typically includes redundant memory cells and CAM cells in addition to a main memory array. The redundant memory cells are used to replace defective cells of the main memory array. The CAM cells are typically used to activate the redundant columns to replace any defective columns of the main memory array. The CAM cells typically comprise flash EPROM cells and are typically arranged into groups in order to be programmed to store addresses of the defective memory columns of the main memory array. The grouped CAM cells are therefore referred to as CAM banks or CAM sets.
A typical prior an arrangement of using the redundant memory columns and the CAM banks to replace defective columns in the main memory array employs a number of the CAM banks, each associated with one redundant column. When a defective column in the main memory array is discovered, the address of the defective column is stored in a CAM bank. A comparison with the stored address is made every time the flash EPROM is addressed to determine whether a cell in the defective column is addressed. If so, the redundant column associated with the CAM bank is activated for the read, programming, or erasure operation of the defective column. If not, the redundant column associated with the CAM bank is not activated.
Advances in technology have led to of multibit state flash EPROMs. For multibit state flash EPROMs, each memory cell stores multiple bits of data. This is done by storing more than two logic levels, typically four or more levels. During a read operation, a number of sense amplifiers are required for each memory cell selected for the read operation. For example, a two-bit state flash EPROM typically requires two sense amplifiers to read the data stored in a selected memory cell. This typically causes the redundancy scheme for a single-bit state flash EPROM to be different from a multibit state flash EPROM.
Typically, for a single-bit state flash EPROM, only one redundant sense amplifier is connected to an activated redundant column. For a multibit state flash EPROM, however, a number of redundant sense amplifiers equal to that of sense amplifiers for a selected column in the main memory array are required for an activated redundant column. Thus, the redundancy scheme for a single-bit state flash EPROM is different from that of a multibit state flash EPROM.